What is the purpose of blood pressure? Obviously, the heart and circulation are assigned the job of supplying all organs of the body with blood. They can do this job only if the basic material—i.e., blood, is available and then, if there is enough pressure to send it on its way. Since the available amount of blood is constant—about 4-5 quarts in adults—the pressure must be able to adjust to the changing requirements of the body. This means it must not fall below a certain minimum level—a condition that can lead to hypotension with its concomitant problems—and that it must be able to react promptly to sudden demands. The cardiovascular system has two simple mechanisms to help it adjust to fluctuating demands for blood by target organs: one is by decreasing or increasing the amount of blood pumped into the circulation, the per minute volume, and the other is by decreasing or increasing the resistance of the muscular arteries through contraction or dilation. This seemingly simple regulatory mechanism is coordinated by a complex control system, which perhaps can best be understood if we first look at the behavior pattern of blood pressure in different everyday situations.